1. Field of the Invention
The invention relates to hot-vulcanizable tread strips for the manufacture of motor vehicle pneumatic tire treads, said strips being produced from a vulcanizable composition. The treads produced therefrom must have all-weather properties, so that the tires can be used year-round.
The invention also relates to block copolymers which can be used in the rubber component without any additional blend component, which rubber component is the basic formulation component of said tread strips, i.e., said copolymers can be used as the rubber component.
2. Description of Background
Tire treads should be highly rated by a series of properties which are important indices of tread performance. These properties are as follows:
(a) High flexibility at low temperatures, i.e., the elastic rubber state is retained even at temperatures below -30.degree. C. (i.e., good cold-weather properties);
(b) High vibration-damping over a wide temperature range, and thus good wet skid resistance, i.e., tread safety;
(c) High wear-resistance, and thus long service life, i.e., economic advantages.
This set of properties has been attained quite imperfectly with the state of the art treads, e.g., treads based on block homopolymers of 1,3-butadiene, as described in Ger. Pat. No. 2,158,575.
Such prior art block homopolymers have the following characteristics:
(1) They are prepared by adiabatic polymerization in an inert organic solvent in the presence of an organolithium catalyst and, as the cocatalyst a Lewis base selected from the group of ethers and tertiary amines, under, of course, conditions of increasing temperatures. During the course of the polymerization, temperatures exceed 155.degree. C.
(2) They consist of macromolecules having main and side chains with recurring units. Their structure is determined by the temperature at formation of the block homopolymer, i.e., by the polymerization temperature. Since the polymerization temperature increases, and consequently therefore the 1,2-polymerization of 1,3-butadiene decreases, the frequency of recurring units produced by the 1,2-polymerization of 1,3-butadiene and thus the frequency of vinyl side groups decreases continuously while the frequency of recurring units produced by 1,4-polymerization of 1,3-butadiene continuously increases. In the chain segments formed at temperatures above 155.degree. C., the frequencies do not change further with temperature.
(3) The macromolecules have sequences of chain segments which have glass transition temperatures, "Tg" (operationally defined infra), which correspond to the conditions of polymerization which change as polymerization progresses. In general the Tg values begin in the range of about -25.degree. to -40.degree. C., and decrease continually to the range of about -80.degree. to -95.degree. C. This trend of decreasing Tg corresponds to the trend of increasing segment mobility of the chain segments in the main and side chains of the macromolecules.
The Tg of a given chain segment corresponds to the Tg of an entire polymer produced under the same conditions. The mechanical loss factor tan .delta. obtained by torsional oscillation of a vulcanized test body, which is obtained from the polymer analogously to the method prescribed in ISO 2322-1975(E) for SBR, is measured according to DIN 53 520, as a function of temperature, yielding a plot of a curve (the "tan .delta. curve") having a maximum. The more or less narrow temperature band corresponding to this maximum is the Tg.
By a general (theoretical or quasi-theoretical) treatment of segment mobility it may be determined that the mobility is a function of the bulkiness of the side groups, and is also a function of their frequency in the segment. In other words, the mobility of a given segment is a function of the types of recurring units and their frequency in the segment.
The order of hindrance to segment mobility of the recurring units in block homopolymers of 1,3-butadiene is as follows: Recurring units obtained by the 1,2-polymerization of 1,3-butadiene hinder more than recurring units obtained by the 1,4-polymerization of 1,3-butadiene.
A typical block homopolymer as described in Ger. Pat. No. 21 58 575 is obtained in the absence of branching agents, under the following conditions: Polymerization in hexane in the presence of 0.04 wt.% n-butyllithium (active catalyst) and 1 wt.% ethylene glycol dimethyl ether (said amounts based on the amount of polymer), with the polymerization begun at 30.degree. C. and ended at 160.degree. C.
The block homopolymer thus obtained may be characterized as follows: The tan .delta. values of a vulcanized test body obtained from the block homopolymer by a method analogous to that of ISO 2322-1975(E) for SBR, are measured according to DIN 53 520, as a function of temperature yielding a plot of a curve having a single maximum. The curve is relatively narrow in shape; i.e., the half-height width of the tan .delta. curve is narrow.